The increasing demand for large power ratings and the physically limited maximum current density of semiconductor devices has led to the development of a parallel connection of semiconductors for high power applications. The parallel connection of power semiconductor components, such as IGBTs, is a widely distributed solution for high power converters. In parallel operation of IGBTs, each parallel connected component receives the switch signal. The switch signal may be originated, for example, from a modulator which decides the appropriate timing for the switching action to take place. The switch signal is divided into control signals for controlling each of the parallel connected components. The intention is thus to operate the switches simultaneously, so that the total current would be divided equally between the components.
However, a current unbalance of the parallel components can occur with the IGBT parallel connection. Current unbalances may occur during the on-state (static operation) and/or switching transients (dynamic operation). In the static operation, current unbalance results in, for example, differing temperatures between the components, which causes the components to age differently and prematurely.
Unbalance in the dynamic operation refers to different behavior of the components during the state changes (e.g., during commutation times at turn-off and turn-on). The simultaneous operation of parallel connected components is desirable so that the changes of current in different current paths are equal in magnitude. Changes in current cause large voltage spikes in the inductances of the current path. It is desirable to have similar current behavior in parallel branches, so that the induced voltages also have similar waveforms. For example, for turn-on, delays in commutation may also lead to situations in which the current rating is exceeded during the commutation when the fastest component takes most of the current intended for parallel operation.
There have been attempts to achieve dynamic current balance between parallel connected power semiconductors by selecting the paralleled components according to certain device parameters (e.g., gate emitter threshold voltage, switching times, on-state voltage, etc.). This approach involves additional manual work carried out by the component manufacturer or by the end user, leading inherently to additional costs.
Another measure to tackle the problem of dynamic current unbalance is to use current balancing networks, as suggested in documents [1], [2], [3], and [7] identified below. However, current balancing networks add costs, volume, weight and losses to the electrical system.
The dynamic current balance may also be achieved by manually setting parameters of the gate units driving the semiconductor components. This settable parameter may be, for example, the gate resistance Rg. Also, this procedure involves manual measurements and may take a long time.
Another possibility is to control the rates of current with the gate units, as suggested in documents [4], [5] and [6] identified below. This solution brings complexity and cost to the system together with higher switching losses since the state changes are prolonged.
Document [7] identified below discloses that the converter can be realized in a symmetric manner regarding stray inductances by proper mechanical layout and design. Such a layout and design leads to additional material, development and manufacturing costs. Further, this kind of design leads to a physical structure, which makes the service of the device complicated and difficult.